Progression of Human Immunodeficiency Virus (HIV) disease to Acquired Immunodeficiency Disease Syndrome (AIDS) involves the loss of CD4+ T cells that provide critical helper activity for humoral and cell-mediated immune responses. Although multiple mechanisms have been proposed to account for the depletion of T cells, the effects of gp120 on cell survival are likely multiple, and involve not only immune cells, but other parenchymal tissues as well. Experiments proposed in this application seek to address the molecular basis and functional consequences of gp120-stimulated CXCR4 signaling. The present application will test the hypotheses: that gp120 ligation of CXCR4 in activated cells initiates PKCa, Lck,p38,and Bim-dependant signal cascade resulting in loss of mitochondria! transmembrane potential with consequent caspase activation and apoptosis (Specific Aim #1);that caspase cleavage intermediates directly transactivate NF-KB-mediated HIV-LTR transcription (Specific Aim #2); and that these events drive CD4 depletion and HIV replication in vivo (Specific Aim #3).